As a thermal resistor type air flow measurement apparatus (gas flow measurement apparatus) for measuring the intake air flow rate taken into the automobile engine, it is well-known that the apparatus is equipped with a main passage in which the fluid to be measured flows and a sub-passage in which part of the fluid flows. The flow detecting elements such as a heating resistor and a thermal sensitive resistor are set in the sub-passage.
Such a sub-passage type flow rate measurement apparatus can stabilize the flow of the gas by utilizing the sub-passage structure. The flow detecting elements can reduce that the flow detecting elements are affected by the changes of the flow velocity distribution, by the pulsation flow and the back flow. Further, it can reduce the contamination of the flow detecting elements, and thereby can reduce the deterioration of quality of the elements. Further, in this apparatus, the flow detecting elements can be mounted easily on the main passage and protected effectively.
When, however, the internal configuration of the sub-passage changed due to adherence of foreign substances, the measuring values of the apparatus will be increased over those of the gas flow measuring apparatus without sub-passage. To improve the function of the sub-passage, various modifications and improvements are incorporated in the shape of the passage, and this tends to complicate the shape of the sub-passage. Especially in order to reduce the pulsating flow and to maintain the measuring accuracy, the sub-passage is designed to have a bent form, with the result that foreign substances tend to deposit in the bent portion. The most probable trouble is that, depending on the mounting angle of the flow measuring apparatus, such a liquid as water will accumulate in the sub-passage, or water remains in the bent portion of the curved sub-passage, thereby causing a flow measurement error.
To solve such problems, technical means are proposed to provide a drain hole for preventing water from remaining in the sub-passage, as disclosed in the Japanese Application Patent Laid-Open Publication No. Hei 07-139414 and Japanese Application Patent Laid-Open Publication No. Hei 09-273950.
The flow measuring apparatus provided with such a drain hole is intended to prevent from troubles caused by remaining of water in the sub-passage or submersion of the flow detecting elements in water.
The so-called leak hole (leak path) including the above-mentioned drain hole is a small hole that does not sacrifice the function of the sub-passage, and prevents from making water remain in the sub-passage. However, the last remaining liquid drop may remain inside the leak hole due to the surface tension.
Such a residual liquid drop such as waterdrop blocks the leak hole, and this causes greater changes in air flow velocity distribution than those when the leak hole is not blocked.
Such a phenomenon causes deterioration of accuracy and performances including changes in the measurement value and increase of output noise.
The present invention has been made to solve these problems. It prevents water and other liquids from adhering and remaining in the sub-passage, and provides a sub-passage that minimizes the flow measuring error caused by liquid drops blocking the leak hole.